In the fabrication of integrated circuits, the exposure of a photoresist to light is an integral process step. The production of high density circuits having submicron dimensions requires that such exposure be accomplished within close processing tolerances. For example, it is important to control the linewidth of the imaged and developed photoresist so that any deviation from the nominal design linewidth over non-planar or reflective features is small, typically less than 10%.
The difficulty in controlling linewidth in high resolution photoresist patterns over reflective topography is well documented. See, for example, D. Widmann and H. Binder, IEEE Trans. Electron Devices, ED22, 467 (1975). When photoresist layers overlaying reflective substrates are exposed using monochromatic light sources, a constructive interference pattern between the normally incident exposing light and light reflected from the substrate is created in the resist. The resulting pattern of optical nodes and antinodes which is normal to the plane of the reflective interface, and repeats along the optical axis, is the cause of localized variations in the effective dose of exposing light. This phenomenon is known in the art as the interference or standing wave effect. Other pattern distortions are caused by light reflected angularly from topographical features and are discussed by M. Bolsen, G. Buhr, H. Merrem, and K. Van Werden, Solid State Tech., Feb. 1986, 83. These distortions are known in the art as reflective notching.
The quantification of the interference effect can be measured by using the swing ratio (SR), set forth by T. Brunner, SPIE, 1466, 297 (1991), EQU SR-4(R.sub.1 R.sub.2).sup.1/2 e.sup.a.sup..sup.D
where R.sub.1 is the reflectivity of the resist/air interface and R.sub.2 is the reflectivity of the resist/substrate interface at the exposing wavelength, a is the resist absorbtion coefficient, and D is the resist thickness. A low swing ratio implies that localized variations in the effective dose of exposing light are small, and thus the exposure dose is more uniform through the thickness of the film. One method to reduce the swing ratio is use of a photoresist or lithographic process which imparts a high numerical value in .alpha. or D, giving a high numerical value to the product of .alpha.D. Other methods for reducing the swing ratio are the use of coatings which reduce the contribution of R.sub.1 or R.sub.2, such as through the use of antireflective layers.
The lithographic techniques for overcoming the problems of forming patterns on reflective topography include dyes added to the photoresists as described in U.S. Pat. No. 4,575,480 to Kotani, et al., U.S. Pat. No. 4, 828,960 to Hertog, U.S. Pat. No. 4,882,260 to Kohara, et al., and U.S. Pat. No. 5,043,243 to Yajima, et al., top surface imaging (TSI) processes, multilayer resists (MLR) with added dyes as described in U.S. Pat. No. 4,370,405 to O'Toole, et al., top antireflective layers (TARL), bottom antireflective layers (BARL) which may comprise inorganic materials or organic materials, and coatings comprising polyamic acids or polybutene sulfone with added dyes.
When a dye is added to photoresist to form an optically sensitive film having high optical density at the wavelength of the exposing radiation, several problems may be encountered. These include sublimation of the dyes during baking of the films, loss of resist sensitivity, difficulties during deep ultra violet hardening processes which are commonly used with novolak comprising resists, thinning of the resists in alkaline developers, and distortion of the resulting relief image. TSI processes require high optical density at the wavelength of the exposing radiation and similar processing difficulties are often encountered. Furthermore, TSI and MLR processes are costly and complex.
Tanaka, et al., have disclosed the use of a TARL, as an optical element overlaying a photoresist layer, however, this approach is not effective in reducing top notching effects from underlaying reflective topography and also requires removal with hologenated solvents prior to the photoresist development step. T. Tanaka, N. Hasegwa, H. Shiraishi, and S. Okazaki, J. Electrochem. Soc., 137, 3900 (1990).
Inorganic BARLs such as silicon require precise control of the film thickness, which for a typical 300 .ANG. thick film is .-+.10 .ANG..T. Pampalone, M. Camacho, B. Lee, and E. Douglas, J. Electrochem. Soc., 136, 1181 (1989). Pampalone has described the use of titanium oxynitrides on aluminum surfaces to reduce reflectivity from 85% to 25%, however, TiNxOy processes require special deposition-equipment, complex adhesion promotion techniques prior to resist application, a separate dry etching pattern transfer step, and dry etching for removal. Horn has disclosed the similar use of titanium nitride antireflective coatings, however, such coatings are often incorporated into the completed semiconductor device as a permanent element, thus TiN coatings are not suitable for use with every photolithographic layer. M. Horn, Solid State Tech., November 1991, 58.
U.S. Pat. No. 4,910,122 to Arnold, et al., discloses organic BARLs comprising polyamnic acids or polybutene sulfones with added dyes. The films derived from the polyamic acid compositions are cured by baking at a temperature of at least 148.degree. C. for 30 minutes. Pampalone has noted that the baking conditions must be carefully controlled to prevent the occurrence of oversized or undersized relief images in the imaged and developed photoresist relief images in the imaged and developed photoresist. Horn has noted that the BARL tended to peel or leave a residual scum. The polyamic acid based BARL is also developed with alkaline developer of the resist. Concurrently, any Al layers which may be in contact with the BARL are attacked by the alkaline developer, which may cause lifting of the BARL and resist layer.
The films derived from polybutene sulfone with coumarin dyes require coating thicknesses of 2.0 .mu.m and baking at 140.degree. C. for 60 minutes. The 2.0 .mu.m thick film of polybutene sulfone may tend to fill in and planarize deep trenches, resulting in localized regions having a film thicker than 2.0 .mu.m, and requiring plasma over etching to remove the film. The use of a 2.0 .mu.m layer with an added 1.0 .mu.m resist layer may exceed the usual depth of focus of less than 2 .mu.m for advanced, higher numerical aperture exposure tools. In addition, long bake times are not compatible with a rapid throughput cluster tool processing strategy, thus, such materials may require additional or separate long coating or baking steps that add process costs. Polybutene sulfone is also thermally unstable at temperatures above about 120.degree. C. and may decompose with out gassing. This may lift the overlying resist during deep ultra violet hardening or A1 etching where the wafer temperature may reach 150.degree. C.
Furthermore the antireflective coating can be used within a bi-layer resist lift-off process in the fabrication of integrated circuit components and other thin film structures such as field effect transistors (FET), conductor patterns and magnetic sensing transducers. These types of devices are well known in the art. For example, U.S. Pat. No. 4,814,258 granted to Tam discloses a bilayer lift-off process utilized for the fabrication of various types of FET/s, and U.S. Pat. No. 5,604,073 granted to Krounbi discloses a bilayer metal lift-off process for forming lead conductors in a magnetoresistive (MR) sensor. In this case the release layer is a suitable material such as polydimethylglutarimide (PMGI), a polymer supplied by the Shipley Company.
Despite these disclosed processes there remains a need for antireflective coatings which may be used to decrease the swing ratio (SR) by lowering the relative reflectivity of the various interfaces in a resist system.